1. Field of the Invention
The present invention relates to a base station, radio terminal and radio communication method, and more particularly, to a technique for reducing interference among users when an OFDMA scheme is used for uplink communication from radio terminals to a base station.
2. Related Art
An OFDMA (Orthogonal Frequency Division Multiple Access) scheme is known as a scheme for multiplexing users by assigning subcarriers divided through OFDM (Orthogonal Frequency Division Multiplexing scheme) modulation to different users (radio terminals). In an uplink radio communication system which multiplexes users according to an OFDMA scheme, user signals overlap one another on the frequency domain caused by a frequency offset of each user, Doppler shift or the like. As a result, interference among users may occur. When interference occurs, an error rate performance degrades, and therefore throughput degrades. As a result, the total throughput of all user terminals connected to a certain base station, that is, system throughput degrades.
To avoid this problem, a method of canceling interference using signals received on the base station side is known, as described in a document (“Multiuser Interference Cancellation Receivers for OFDMA Uplink Communications with Carrier Frequency Offset” (R. Fantacci, D. Marabissi, S. Papini, pp 2808 to 2812, IEEE Communications Society Globecom 2004)). However, this method requires complicated calculations to cancel interference. Furthermore, interference may not be totally canceled.
In contrast, as a simple way of reducing interference, a scheme of constantly arranging null subcarriers among users may be considered. This can be considered equivalent to a case where guard bands are provided among users under an FDM scheme. When this scheme is used, interference among users does not occur unless a frequency offset exceeding the widths of pre-arranged null subcarriers or a Doppler shift occurs. For this reason, it is possible to prevent deterioration of system throughput due to interference. However, when no interference occurs, system throughput in this scheme reduces compared to a case where null subcarriers are not arranged.
Thus, the conventional method requires complicated processing when interference is canceled. Furthermore, there is also a problem that system throughput degrades when interference does not occur.